There are many important reasons for pursuing tunable filter imaging, as demonstrated by the TTF since the mid 1990s at the Anglo-Australian 3.9m and William Herschel 4.2m telescopes. Conventional imaging has a major limitation in that images are taken sequentially which is not ideal at even the best sites. Even small detector, instrument or atmospheric variations lead to systematic error between images. A better approach is a multi-band camera so that different bands are observed in parallel. However, even this is not ideal in that the optical path is different for each filter. A tunable filter provides a powerful alternative since band switching can be linked to charge shuffling with the CCD. This is a truly differential technique which leads to much smaller systematic errors than is possible with conventional imaging.